A pretty good story.
I liked Sokka and Toph's dialogue.
The detailed description of Katara's undressing was a nice touch.
I really liked how you described Katara's bowel movements.
Starting of the first part of her evacuation "It flowed smoothly and easily from her colon" made it sound to me like she really needed to go.
I sure would like someone to draw Katara on the pot, preferably without a view of her "moistening pussy".